This application claims the priority benefit of Taiwan application serial no. 89123050, filed Nov. 2, 2000.
1. Field of Invention
The present invention relates to an alignment method for photolithographic processing. More particularly, the present invention relates to a method of forming alignment marks after ion implantation to serve as aligning markers for subsequent processes.
2. Description of Related Art
Photolithography is one of most important processes for fabricating semiconductor circuits. Photolithographic processes are used in the transfer of a pattern onto a thin film and the fabrication of a mask for ion implantation. In general, photolithographic processing is conducted many times in the production of a semiconductor circuits. In a photolithographic step, one critical factor is pattern alignment. A wafer to be processed not only must be aligned and leveled relative to the stepper machine performing the patterning, the wafer must also be properly aligned relative to the previous pattern. Conventionally, an alignment mark or marks are used for alignment before carrying out photo-exposure. Moreover, an after development inspection (ADI) is conducted after exposure and chemical development.
Alignment markers are often formed outside the device region at the same time as thin film such as an insulation layer or a conductive layer is patterned. Typically, the alignment marker has a circular or linear shape. Optical alignment is generally achieved by aligning with the protruding or recess alignment markers on different thin film layers.
FIGS. 1A through 1D are schematic cross-sectional views showing the progression of steps for forming conventional alignment markers before conducting an ion implantation. Conventional ion implantation is carried out using a patterned photoresist layer as a mask. In this embodiment, the use of alignment pattern is explained with respect to performing an ion implantation. As shown in FIG. 1A, a silicon substrate 100 having a device region for forming semiconductor device is provided. The substrate 100 also has an alignment mark region normally located close to the scribe line of a wafer for forming alignment markers. A photoresist layer 102 is formed over the substrate 100. The photoresist layer contains openings that expose ion-implant regions within the device region and alignment markers for machine positioning.
As shown in FIG. 1B, an ion implantation is carried out using the patterned photoresist layer 102 as a mask to from a doped region 104 such as the source/drain region of a metal-oxide-semiconductor (MOS) transistor. Meanwhile, another doped region 104 is also formed in the alignment mark region to serve as an alignment marker. As shown in FIG. 1C, the photoresist layer 102 is removed.
As shown in FIG. 1D, subsequent photolithographic processing is conducted. Another patterned photoresist layer is formed over the substrate 100. The alignment marker 106 formed during patterning must be inspected for accuracy with the previous alignment marker. However, after a conventional ion implantation, only an alignment marker doped region remains in the alignment mark region. Since no step height or protrusions are formed on the wafer surface, gauging alignment by detecting optical diffraction signal is almost impossible. Hence, judging alignment accuracy of the alignment marker 106 relative to the marker created during ion implantation is difficult.
Accordingly, one object of the present invention is to provide a method of forming alignment marker for photolithographic processing. The method is capable of producing an alignment marker that has a step height for gauging optical alignment after a subsequent process.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method for forming an alignment marker on a substrate after ion implantation. A substrate that includes a device region and an alignment mark region thereon is provided. A first patterned photoresist layer is formed over the substrate. The first patterned photoresist layer includes an alignment marker within the alignment mark region and an ion implantation pattern within the device region. Using the first patterned photoresist layer as a mask, an ion implantation is carried out to form a plurality of doped regions. A second patterned photoresist layer that exposes the alignment marker is formed over the ion-implant pattern of the first patterned photoresist layer. Using the alignment marker as a mask, the substrate is etched to form a plurality of recess regions in the substrate.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.